Balanced and Unbalanced Forces

Balanced and Unbalanced Forces

Middle School

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Learning Objectives

• Describe a force and its two key properties: size and direction.

• Differentiate between balanced and unbalanced forces based on their net force.

• Calculate the net force on an object from forces in the same or opposite directions.

• Explain how balanced and unbalanced forces affect the motion of an object.

Key Vocabulary

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Force

A push or pull on an object that causes a change in the object's motion.

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Newton

The standard unit for measuring the size or magnitude of a force, abbreviated as N.

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Net Force

The overall force on an object, which is the combination of all forces acting on it.

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Balanced Forces

Equal forces acting on an object in opposite directions, resulting in no change in motion.

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Unbalanced Forces

Unequal forces that cause a change in an object’s motion, such as its speed or direction.

Common Forces on Earth

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Gravity

• ​Gravity is the force that pulls objects toward each other.

• ​​On Earth, gravity pulls everything toward the center of the planet.

• ​An apple falls to the ground from a tree because of gravity.

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Friction

• ​Friction is a force that resists motion between two touching surfaces.

• ​​This force occurs when two objects are rubbing against each other.

• ​Friction helps a sliding book on a table to slow down.

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Air Resistance

• ​Air resistance is a type of friction that acts upon objects.

• ​​It happens at the moment an object is moving through air.

• ​A parachute makes use of this to slow a person’s fall.

Multiple Choice

Which of the following best describes friction?

• A force that speeds up objects as they move

• A force that resists motion between two surfaces that touch

• A force that pulls objects toward the Earth

• A force that only works when objects are far apart

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Combining Forces: Net Force

Forces in the Same Direction

• The net force is the combination of all forces acting on a single object.

• When forces are in the same direction, you add them together to find the net force.

• The object will move in the same direction as the combined forces.

Forces in Opposite Directions

• The net force determines how an object’s motion will change from all forces involved.

• When forces are in opposite directions, you subtract the smaller force from the larger one.

• The object will move in the direction of the greater force.

Multiple Choice

You and your dad are moving a couch. Your dad pushes with a force of 35 N to the right, and you push with a force of 25 N to the right. What is the net force on the couch?

• 60 N to the left

• 10 N to the right

• 50 N to the right

• 60 N to the right

Balanced vs. Unbalanced Forces

• Balanced forces are equal and opposite, so the net force is zero.

• Unbalanced forces are unequal, resulting in a net force greater than zero.

• A change in motion is always caused by an unbalanced force.

Multiple Choice

What is the primary difference between balanced and unbalanced forces?

• Balanced forces cause a change in motion; unbalanced forces do not.

• Only balanced forces are measured in Newtons.

• Unbalanced forces only apply to objects at rest.

• Unbalanced forces cause a change in motion; balanced forces do not.

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Effects of Unbalanced Forces in Action

• Unbalanced forces cause changes in motion, like speeding up or slowing down.

• An object will always move in the direction of the greater force.

• A baseball slows as it rises and speeds up as it falls due to gravity.

• When it stops, forces become balanced and its motion does not change.

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Multiple Choice

According to the example, why does a baseball slow down as it rises into the air after being hit?

• Because it is changing direction.

• Because the forces on it become balanced.

• Because it runs out of the force from the bat.

• Because of the unbalanced force of gravity.

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Common Misconceptions About Forces

Misconception	Correction
If an object is not moving, there are no forces acting on it.	An object at rest has balanced forces acting on it.
An object in motion must have an unbalanced force acting on it.	Objects at a constant speed have balanced forces. Unbalanced forces cause acceleration.
Force is a property that an object possesses or 'has'.	A force is an interaction—a push or a pull—between two objects.

Multiple Choice

A rolling soccer ball gradually slows down and stops on a grassy field. Which statement best explains why the ball's motion changes?

• The ball stops because its net force becomes zero only when it stops moving.

• The initial force from the kick wears off, causing the ball to stop.

• The forces acting on the ball are balanced, which causes it to slow down.

• The forces of friction and air resistance are unbalanced forces that oppose the ball's motion.

Multiple Choice

Two students push a heavy box in opposite directions. Student A pushes to the right with a force of 50 N, and Student B pushes to the left with a force of 70 N. What is the net force and the resulting motion of the box?

• The net force is 120 N to the left, and the box will move to the left.

• The net force is 20 N to the left, and the box will move to the left.

• The net force is 20 N to the right, and the box will move to the right.

• The net force is 0 N, and the box will not move.

Multiple Choice

To win a game of tug-of-war, your team must pull the rope in your direction. How can you use your understanding of forces to develop a winning strategy?

• Your team must apply a force greater than the opposing team's force to create an unbalanced force.

• Your team must ensure the forces are perfectly balanced with the other team.

• Your team must reduce the friction between your shoes and the ground.

• Your team should pull with a force of exactly 0 N to win.

Multiple Choice

A car is traveling at a constant speed of 60 mph on a straight highway. What evidence supports the conclusion that the forces acting on the car are balanced?

• The engine is pushing the car forward, which is the only force acting on it.

• The car is not accelerating; its speed and direction are unchanging.

• The car is moving, and all moving objects have unbalanced forces.

• The force of air resistance is greater than the force from the engine.

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Summary

• A force is a push or pull with both size and direction, measured in Newtons (N).

• The net force is the combination of all forces acting on an object.

• Balanced forces have a net force of 0 N and do not change an object's motion.

• Unbalanced forces have a net force > 0 N and cause an object to accelerate.

Poll

On a scale of 1-4, how confident are you about explaining the difference between balanced and unbalanced forces?

• 1

• 2

• 3

• 4